Bug 102622

The recommended way to deal with libraries that might not have working .pc metadata appears to be this:

> [Alternatively, you may set the environment variables $1[]_CFLAGS
> and $1[]_LIBS to avoid the need to call pkg-config.
> See the pkg-config man page for more details.])

and in the development branch of dbus, we've documented EXPAT_CFLAGS and EXPAT_LIBS as the way to avoid relying on pkg-config metadata for archaic or wrongly-installed versions of Expat (which seems to include SuSE's mingw32-expat, Bug #102613).

However, in a cross-compilation environment with a pre-configured cross-compiler, it's entirely possible that EXPAT_CFLAGS can legitimately be empty: for example, on Bug #102613, <expat.h> appears to already be on the cross-compiler's default search path, so no extra -I flag should be required.

The pkg-config macro ends up doing this:

> m4_define([_PKG_CONFIG],
> [if test -n "$$1"; then
>     pkg_cv_[]$1="$$1"
>  elif test -n "$PKG_CONFIG"; then
> ... rely on pkg-config ...

This means that if $1 (in this case EXPAT_CFLAGS) is set, but to an empty value, then we require .pc metadata.

Perhaps the macro should have this pseudo-patch:

 m4_define([_PKG_CONFIG],
-[if test -n "$$1"; then
+[if test -n "${$1+set}"; then
     pkg_cv_[]$1="$$1"

so that the decision is really between set vs. unset, rather than between (set && non-empty) vs. (unset || empty).